Generally, a condenser microphone widely used in mobile communication terminals or mobile audio devices includes a voltage bias component, a pair of diaphragm/back plate for forming a capacitance C that changes in correspondence with sound pressures, and a field effect transistor (JFET) for buffering output signals. Such is a traditional type condenser microphone is completely formed as a single assembly by inserting a diaphragm, a spacer ring, an insulation ring, a back plate, and an electric current application ring into a single case in the stated order, inserting a PCB, on which circuit devices are mounted, into the case, and bending an end of the case toward the PCB.
Recently, a semiconductor fabrication technique using micromachining methods has been used for improving the integration of fine devices. By using this technique, a so-called micro electro mechanical system (MEMS), μm-sized ultra-small sensors, actuators, and electro-mechanical structures may be fabricated by using micromachining methods, and more particularly, integrated circuit methods, in a semiconductor fabrication process. In a MEMS chip microphone fabricated by using such micromachining methods, traditional microphone components, such as a diaphragm, a spacer ring, an insulation ring, a back plate, and an electric current application ring may be miniaturized, multi-functionalized, and densely integrated via ultra-high precision fabrication methods for improved stability and reliability.
FIG. 9 shows a structure of a general MEMS chip 10 used in a silicon condenser microphone. Referring to FIG. 9, the MEMS chip 10 has a structure in which a back plate 13 is formed on a silicon wafer 14 by using a MEMS technique and a diaphragm 11 is formed on the back plate 13 by interposing a spacer 12 therebetween. A sound hole 13a is formed in the back plate 13, and a rear space of the back plate forms a back chamber. The MEMS chip 10 is fabricated by using a general micromachining method and a semiconductor chip fabrication method.
FIG. 10 is a lateral sectional view of a conventional silicon condenser microphone 1 embodied by using a MEMS chip 10. Referring to FIG. 10, the conventional silicon condenser microphone 1 is assembled by mounting the MEMS chip 10 and an ASIC chip 20 on a PCB 40 and inserting the PCB 40 into a case 30 in which a sound hole 30a is formed.
As shown in FIG. 10, a back chamber 15 of the conventional silicon condenser microphone 1 is formed by the MEMS chip 10. However, the MEMS chip 10 is a very small-sized semiconductor chip, and thus, the back chamber 15 formed by the MEMS chip 10 is very narrow. As a result, air resistance occurs due to a significant back stream and a vibrating force of a diaphragm deteriorates, and thus, the sound quality is (sensitivity) of the conventional silicon condenser microphone 1 deteriorates.